Recently, there are lots of increasing interests on multiferroic materials. Multiferroic materials can be defined as materials which have at least two properties among (anti)ferroelectric, (anti)ferromagnetic and ferroelastic properties at the same time. For example, a multiferroic material having ferroelectric and ferromagnetic properties at the same time may change its magnetic property with an external electric signal, or change its electric property with an external magnetic signal by coupling the electric property of the ferroelectrics and magnetic property of the ferromagnetics.
Unlike prior art which prepared elements by coupling at least two materials having different properties, such characteristics of multiferroics enable new inventions such as a single material device equipped with various functions. Accordingly, recently, there are a great number of reports related to multiferroic materials. However, only few materials are known to have multiferroic properties.
Among them, currently, as a material having an orthorhombic crystalline structure, the orthorhombic manganites represented by the following formula RMnO3 (R=La, Pr, Nd, Sm, Eu, Gd, Tb or Dy) are investigated actively. Said manganites have orthorhombic structure in bulk phase. In particular, among the manganites in bulk phase, the crystalline structure of TbMnO3 is described in detail in FIG. 1. In bulk phase, the TbMnO3 has orthorhombic properties as described in FIG. 1. The TbMnO3 is a multiferroic material having ferroelectric and antiferromagnetic properties at the same time, and there is a strong coupling between said ferroelectric and antiferromagnetic properties. For example, the TbMnO3 shows a flopping of polarization direction when magnetic field is applied. The material shows a ferroelectric property at a temperature in the range of about 21˜27 K, and an antiferromagnetic property at a temperature in the range of 41˜43 K.
FIG. 2 is a phase diagram explaining the magnetic properties of orthorhombic manganites. As shown in FIG. 2, among the manganites represented by the above formula, the manganites present multiferroic properties only when R is Gd, Tb, and Dy, and it does not present multiferroic properties when R is Nd, Sm, and Eu. However, even when R is Gd, Tb, and Dy, there is a problem that the ferroelectric transition temperature Tc and remnant polarization value PR are too low to be applied to actual elements (For example, it can not be applied in a process using liquid nitrogen).
However, it is possible to change the physical properties of the specific material by changing the crystal structure of the material. This is because the band structure, orbital, phonon, etc. greatly change as the crystalline structure changes even when the chemical stoichiometry is the same. Bosak et al. reported in Cryst. Eng. 5, 355 (2002) and Chem. Mater. 15, 2632 (2003) the growth of RMnO3 (R=Sm, Eu, Gd, and Dy) layer using YSZ (111) substrate. In particular, they showed that the RMnO3 (R=Sm, Eu, Gd or Dy) layer could be grown to have hexagonal structure. However, they have not showed electric or magnetic properties of epitaxially stabilized hexagonal RMnO3 layer. Thus, it has not been revealed whether the epitaxially stabilized hexagonal RMnO3 layer has multiferroic properties or not. Also, they did not disclose a structure comprising a bottom or top electrodes which are essential for capacitor structure as well as a stabilized hexagonal RMnO3 layer.
Therefore, the present inventors completed the present invention considering that multiferroic properties can be provided to the material, or that the multiferroic properties can be enhanced by substituting the crystalline structure of the orthorhombic RMnO3 material for a hexagonal crystalline structure that is not present in the natural world.